An elliptical and divergent laser beam generated from a gain-guided laser diode is illustrated in FIG. 1. The virtual source point 1a from longitudinal cross section beam is located at the output facet of the laser diode, but the other virtual source point 2a from transverse cross section beam is displaced a distance behind the facet. The distance between 1a to 2a is defined as inherent astigmatism. Further, the divergence angle 3a of longitudinal ray 1 is relatively greater than divergence angle 3b of transverse ray 2. The ratio of the divergence angles typically ranges from 2:1 to 6:1 for various semiconductor laser diodes. Because there are two virtual source points along the optical axis 3, collimation of the elliptically diverging beam into a parallel beam cannot be achieved using a rotationally symmetric lens. For systems that require high resolution, it is important to correct the astigmatism in order to ultimately obtain the smallest (i.e., diffraction-limited) spot. Such correction can allow, for example, maximum storage density in an optical read/write head. Prior art approaches to this problem have used multiple optical elements such as cylindrical lenses and prisms as described in U.S. Pat. Nos. 4,643,538, 4,318,594, and 5,239,414, the disclosures of which are herein incorporated by reference. However, these methods increase both the total optical path and the size of the optical system and, further, introduce complexity in alignment procedures.
In many applications, the laser diode beam must not only be collimated but must also be corrected to a circular, rather than elliptical, cross-sectional shape. Prior art approaches for collimation and circularization have involved the conventional use of apertures as well as the use of external anamorphic prism pairs, micro-lenses, or multiple optical elements with graded indices of refraction as described in U.S. Pat. Nos. 4,734,906, 5,553,174, 4,810,069, and 5,251,060, the disclosures of which are herein incorporated by reference. Each of these approaches, however, suffers one or more drawbacks. The use of apertures reduces optical efficiency. In addition, anamorphic prism pairs are costly and increase both system size and assembly difficulties. Further, graded-index materials require additional processing to achieve the graded-index profile.
Other prior art approaches addressing circularization and collimation of elliptically diverging beams have focused on the use of single correcting optical elements. These approaches include the use of single lenses with aspherical or cylindrical surfaces and the use of graded-index single rods or fibers as described in U.S. Pat. Nos. 5,159,491, 5,572,367, and 4,575,194, the disclosures of which are herein incorporated by reference. These prior art approaches, however, have not realized the simultaneous correction of beam astigmatism, beam divergence, and non-circular beam shape in a single element.
Accordingly, an object of the present invention is to provide a single optical element which achieves a collimated and de-astigmatism beam profile corrected to a circular cross-section.